The Internet is used to provide a variety of different forms of communication between users. Different communication systems may be operated by different software providers, wherein different applications may be implemented at a user device to enable a user of the user device to communicate over the different communication systems. The different communication systems may be managed and operated independently.
For example, a first software provider may offer a first voice and/or video over internet protocol (“VoIP”) client, which may become very popular with a large number of users communicating with each other via this VoIP client over a first communication system. Similarly, a second software provider may offer a second VoIP client (or some other application allowing communication across the Internet), which may also become very popular with a large number of users and which enables communication over a second communication system. To gain the benefit of both these services, a user runs both the two VoIP clients separately at his user device. Furthermore, the user maintains separate contact lists for each of the two services. A user is identified in each communication system using a respective user identifier (ID). An ID of a user in a first communication system may be different to an ID of the user in a second communication system. The IDs may, for example, be email addresses or other user names which are unique to the user within the respective communication systems, thereby allowing the users to be uniquely identified within each of the communication systems.
In order for two users to communicate with each other across the communication systems, the communication systems may require that there is a contact relationship between the two users. For example, each user may have a respective contact list of his contacts within each of the communication systems. When users are contacts of one another within a communication system, they are authorised to communicate with one another over that communication system. However, when users are not contacts of one another within a communication system, they are not authorised to communicate with one another over that communication system. In one example, two users are not contacts of one another within a first communication system, but are contacts of one another within a second communication system. The two users can therefore communicate with each other over the second communication system. However, before the two users can communicate with each other over the first communication system they would first add each other as contacts within the first communication system, such that communication between the two users is authorized to proceed over the first communication system.